Above and partly surrounding the subcortical centers are the large brain and the cerebellum or small brain. The ganglion cells of the neurons contained in the cerebrum and cerebellum are all located near the surface or cortex. There seems to be a peculiar advantage—not yet perfectly understood—in having the gray matter spread out over the surface of the cerebrum and cerebellum in as thin a layer as possible. To this end the surface of the cerebrum is much increased by the formation of large folds, separated by deep fissures (see figure 10). In the cerebellum the folds are more numerous and exceedingly fine, and they do not have the appearance of being the product of fissuration. The surface of the cerebrum is estimated to be equal to a square with a side eighteen inches long. Without the fissures the surface would be only about one third of this. The mixture of ganglion cells and fibers making up the gray matter of the brain is illustrated in figures 11 and 12. Both are sections of the cortex of the cerebrum. In figure 11 the cell bodies alone are stained and thus made visible; in figure 12 the fibers alone are stained.
From what has been said thus far it is clear that certain areas of the cortex must be connected with certain groups
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Fig. 11.—Section of the Cerebral Cortex. Only the cell bodies are stained. |
Fig. 12.—Section of the Cerebral Cortex. Only the fibers are stained. |
of sensory points or motor points of the body much more directly than with others. This is confirmed by histological, pathological, and experimental investigations. For the eyes and the ears, for the muscles of arms and legs, hands and feet, even the several fingers and toes, the corresponding areas of the cortex—that is, the areas with which there is direct connection—are definitely known. [Figure 13] conveys an idea of the relation between certain parts of the brain and the sensory and motor organs of the body.